This invention relates to an electric motor control and variable speed drive which includes a motor housing, a constantly driven rotor nonrotatably connected with a disk flywheel and an intermittently driven shaft on which a clutch disk and a brake disk are nonrotatably mounted. The present invention more particularly relates to such a drive in which the clutch disk can be coupled with one frictional surface of an axially displaceable magnetic armature disk, this disk being mounted on the disk flywheel, when a clutch winding is energized, and whereby the brake disk can be brought into mesh with one frictional surface of an axially displaceable brake armature disk mounted in the motor housing, when a brake winding disposed in a brake bearing plate is energized.
Similar known drives, for example the drive disclosed in the U.S. Pat. No. 3,750,781 to Lengsfeld issued Aug. 7, 1973 and entitled "Electric Motor with Built-In Electro-magnetic Disk Clutch and Brake" have the fundamental advantage that the inertia of the parts which are only intermittently driven, namely, when the clutch winding is energized, is relatively small, thereby achieving short acceleration and/or short braking periods. This is especially advantageous in cases where a relatively high switching frequency is utilized, as is the case for example when such drives are used as positioning drives for sewing machines in the sewing industry.
In the drive known from the afore-mentioned U.S. Pat. No. 3,750,781, the clutch winding is also disposed in a bearing plate. When the clutch winding is energized, the magnetic armature ring which is fastened to the disk flywheel and is axially spring loaded is drawn to the clutch magnet constituted by the clutch winding and the bearing plate, thus engaging the clutch lining on the clutch disk. The brake operates on a similar principle. Since the clutch winding and brake winding are disposed in the two bearing plates of the housing, the two bearing plates must be magnetically insulated from one another to prevent any undesirable mutual influences. As a result, and because of the nature of the design of the clutch and brake disks, i.e., because of the mechanical transmission of the force of friction, relatively high electrical power is required to excite the clutch winding and/or the brake windings.
In the drive known from German Offenlegungsschrift No. 2,105,959, the clutch disk and/or brake disk are themselves made of ferromagnetic material and are drawn against a brake lining disposed between the brake winding and the brake disk when the brake winding is energized and drawn against the disk flywheel disposed between the clutch winding and the clutch disk when the clutch winding is energized. As a result of the unfavorable magnetic flux, high electrical power levels are required, especially to excite the clutch winding.